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Full text of "Magnetism of Carbonados"

Magnetism of Carbonados. G. Kletetschka, P. T. Taylor and P. J. Wasilewski, GSFC/NASA, Greenbelt 
20771, Maryland, USA; gunther@denali.gsfc.nasa.gov. 



Introduction: Carbonados are porous polycrystal- 
line (with crystal sizes up to 100 (xm) diamonds [1]. 
Carbonado is found only in alluvial deposits in Bahia, 
Brazil and in the Central African Republic (CAR). 
Alluvial deposit host is 1.5 Ga while the carbonados 
are between 2.6 - 3.8 Ga [2]. The process of fusing 
the carbonado microcrystals together is not fully un- 
derstood, partly due to fact that the origin of these 
carbonado is not known. 

Models of origin: Several modes of origins are 
proposed for carbonado [3]. First, a crustal origin. 
Carbonados have a light carb on and hel ium isotopic 
"■' si'^aturenP3irTEey contam an enrichranfljrtEr 
light rare-earth elements (REE) [4,6], Carbonados 
have tightly trapped atmospheric noble gases [7] and 
contain an evidence of high He content despite the 
carbonado expected depletion of He at mantle tem- 
peratures [8], Carbonados have high porosity incom- 
patible with high pressure mantle conditions [9]. 

Second, a mantle origin is proposed based on 
similar REE pattern to kimberlites [10,11]. The pres- 
ence of nitrogen platelet (by IR spectra) indicates high 
temperature origin [10,12] and syngeneic inclusions 
of rutile, ilmenite, and magnetite indicates high pres- 
sure and high temperature conditions consistent with 
mantle origin as well [13]. 

Third, it is proposed that carbonado diamonds are 
a result of early impacts into crustal rocks. This is 
indicated by the rare and controversial occurrence of 
high pressure diamond polymorph, londsdaleite, fre- 
quently found in diamonds from meteorite impact sites 
[14], and by observation of planar deformation fea- 
tures, possibly indicating shock events [15]. 

Finally, it is suggested that carbonados have an 
extraterrestrial origin, as indicated by a long term 
annealing based on observation of a zero-phonon line, 
attributed to paired nitrogen atoms in association with 
a vacancy [9]. 

Previous magnetic work: Collinson [16] carried 
out a magnetic properties study and his results are 
summarized. NRM intensity (natural remanent mag- 
netization) (1-10 10 -5 Am 2 kg ] ) is stable with very 
small decrease (-10%) after demagnetization to 100 
mT. The NRM directions show a steady migration to 
stable primary endpoints with irregular intensity decay 
curves. When measured in high magnetic field gradi- 
ent as a function of temperature, carbonados showed a 
low magnetic discontinuity at 120 - 160 C. An initial 
magnetic susceptibility was in a range 5 10" 8 m 3 kg _1 . 



During acquisition of an isothermal remanent mag- 
netization (IRM) there is no indication of saturation 
being reached at 800mT applied field. Js/NRM ratio is 
between 10 - 100 as oppose to a typical range of ter- 
restrial material which is in between 100-1000 [17]. 

Material and method: We obtained 20 samples of 
carbonados from the CAR. They are of variable sizes, 
colors and morphology. Samples show pores of vary- 
ing number and size which decrease in quantity where 
the surface is glassy smooth. All samples were cleaned 
with the ultrasound cleaner before any procedures 
started. NRM and Saturation Isothermal Remanent 
"Magnetizatibn (SlRM) of carbonados were^measured 
with the Superconducting Rock Magnetometer (SRM). 
Hysteresis properties of strongly magnetic samples 
were measured with the vibrating sample magne- 
tometer (VSM). Magnetic fields up to 2 T were used. 
Surfaces of all samples were characterized by optical 
microscopy and Scanning Electron Microscopy. Sam- 
ples with a detectable magnetic signature were cleaned 
in a mixture of 1 part 50% HF acid and 1 part 6N so- 
lution of nitric acid for 24 hours to remove surface 
contamination. Than they were washed in 6N solution 
of hydrochloric acid renewing it every 3 days. During 
the acid change the samples were cleaned ultrasoni- 
cally as well. Mass and SIRM were measured after 14, 
46, 64 days. Even after 64 days samples still kept re- 
acting to the HCL as indicated by the yellow colora- 
tion of the acid towards the end of the 3 day period, 
when the acid was due to change. However, the mag- 
netic signature dropped significantly. At the end of the 
acid treatment samples were characterized by SEM 
microscopy. 

Magnetic Results: Table 1 summarizes the mag- 
netic characteristic before acid treatment. 



pte 


wdght 


NRM 
[AmVl 


SIRM 


REM 

NRM/SIRM 


He 




Dl 


238.2 


0.000004 


0.000101 


0.0431 


0.02 


0.0002 


D2 


143.1 


0.000011 


0.000107 


0.0991 




0.0002 


D3 


233.3 


0.000005 


0.000524 


0.0086 






DJ 


214.6 


0.000003 


0.000039 


0.1367 




0.0001 


D4 


160 .5 


0.O0O099 


0.000393 


0.2522 


0.0015 


0.0057 


rw 


170.1 


0.000008 


0.001067 


0.0075 






D7 


235.0 


0.00OOO4 


0.000332 


0.0109 






D8 


197.7 


0.000007 


0.O0O020 


0.3304 






D9 


213.6 


0.000004 


0.000159 


0.0222 


0.03 


0.0004 


D10 


175.1 


0.000006 


0.001067 


0.0061 






D11 


3123,4 


0,000004 


0.005607 


0.0007 


0.031 


0.0105 


D12 


3456.8 


0. 000003 


0.000200 


0.0137 


0.10 


0.0002 


DJ3 


618.8 


0. 000002 


0.0O023O 


0.0087 






DM 


467.4 


0.000002 


0.000736 


0.0031 


0.1 


0.0015 


D1J 


410.2 


0.000049 


0.000542 


0.0911 


0.0005 


0.012 




616.8 


0.000002 


0.000154 


0.0143 








519.9 


0.000002 


0.000104 


0.0227 






D18 


297.9 


0.000003 


0.000011 


0.2940 






D19 


405.0 


0.00O002 


O.OO0I51 


0.0)65 






DM 


394.8 


0.000020 


0.000183 


0.1080 




0.0003 


Tab 


lei: 













NRM values ranged between 0.1-10 10" 5 Am 2 kg _1 
which is consistent with the measurements by [16] 



Magnetism in Carbonado Diamonds: G. Kletetschka, et al. 



who reports the same range of NRM values. Satura- 
tion magnetization ranged in between 1.0-500 10" 5 
Am 2 kg" 1 and REM values between 0.001-0.3. The 
hysteresis parameters were measured only on a limited 
number of samples, due to lower sensitivity of VSM 
Loops were often constricted, indicating two magnetic 
components with different coercivities. One loop 
(sample D15) resembled native iron based on a large 
saturation field. All of the samples showed a diamag- 
netic component (-2 10" 2 A m 2 kg _1 ), due to the dia- 
mond matrix. 

Acid cleaning resulted in loss of both saturation 
remanence (Fig. 1) and mass (Fig. 2) of the samples. 



Acid effect on mass (m ) 




0.95 



Fig. 1: 



d3 d6 dll dl2 dl3 dl4 (115 (120 
sample 

Acid effect on saturation magnetization (Ms ) 



1.0 



0.80 



0.60 



0.40 



0,20 



0.0 







, .■■!■. M!-..,!-..^... 


■ Ms original 

■ Ms after 14 days 
B Ms after 46 days " 
H Ms after 64 days 


liBl 1 n~ 



Fig. 2: 



d3 d6 dll dl2 dl3 dl4 dl5 d20 
sample 





%f.V 












4.0 


: ■ 






. 


1 










■ 


s 


3.0 








. 


u 

1 












8 


2.0 


- 




i 


- 


I 






■ 






1.0 






■ 


- 












■ ■ 




0.0 








, , 



100 



10" 



1000 
Sample man [mg] 

Fig^., _ ,..^__ .___ 

A much greater percent of the magnetic signal was 
lost compared with mass loss. Amount of mass loss 
from the sample was dependent on size (Fig. 3). 

This is probably due to increase of the surface area 
vs. volume with decreasing carbonado size. Thus the 
acid desolves smaller grains more efficiently than 
large ones. However, the magnetization loss is essen- 
tially independent on the size of the original sample 
(Fig 4). 




1000 
Sample mass [mg] 

Fig. 4: 

This independence in size and large amount of 
magnetization lost indicate that most of the magnetic 
material is concentrated on the carbonado surface. 

Because magnetism of Carbonado comes from the 
surface, indicating contemporary formation of both the 
surface and magnetic carriers and/or some kind of 
later chemical contamination, the interior of car- 
bonado must be relatively free of magnetic phases. 

References: [1] Trued, L. F. et al. (1971) Am Miner., 
59, 1252-1268. [2] Ozima, M. et al (1997) GCA, 61, 369- 
376. [3] Haggerty, S. E. (1999) Science, 285, 851-860. 
[4] Kamioka, H. et al. (1996) Geochem. J., 30, 189-194. [5] 
Burgess, R. et al. (1998) Chem. Geol 146, 205-217. [6] 



Shibata, K. et al. (1993) Min. Mag,, 57, 607-611. [7] 
Ozima, M. et al. (1991) Nature, 351, 412414. [8] Zashu, S. 
et al. (1995) GCA, 59, 1321-1328. [9] Haggerty, S. E. 
(1998) Proc 5th NIRM Int. Symp. Adv. Mat, 39-42.[10] 
Kagi, H. et al. (1994) GCA, 58, 2629-2638. [11] Gorshkov, 
A. I. et al. (1997) GeoL Ore Dep, 39, 229-236. [12] 
Shelkov, D. et al. (1997) GeoL Geojl, 38, 315-322. [13] 
Gorshkov, A. L et al. (1996) GeoL Ore Dep., 38, 114-119. 
[14] Smith, J. V. et al. (1985) Geology, 13, 342-343. 
[15] De, S. et al. (1999) EPSL, 164, 421-433. [16] Collin- 
son, D. W. (1998) EPSL, 161, 179-188. [17] Wasilewski, P. 
J. (1977) PEPl 15, 349-362.